In the not-too-distant past, there were relatively few telecommunications networks. These telecommunications networks supported a limited assortment of end-user equipment with electro-mechanical switching systems interconnected with copper wire. In recent years, however, there has been an explosive growth in demand for telecommunications services that employ a wide variety of new end-user equipment This demand quickly exceeded the capacity of the copper networks and electro-mechanical switching systems. To meet this demand, service providers old and new turned to optical fiber systems interconnected by computer-controlled (electronic) switching systems.
Today, high-capacity optical fiber networks are increasingly exploited in both public and private telephone and data networks. In the early stages of optical fiber networks, however, deployment was limited to high-revenue-generating applications. This limited deployment was due to communications-equipment manufacturers making network components using unique, proprietary architectures. The result of which, of course, was that the network components from one manufacturer did not work with other manufacturers' network components. An operating company implementing an early optical fiber network had to purchase most, if not all, of its network components from one manufacturer.
In order to provide inter-operability among components from the various manufacturers (and thus lower costs to the operating companies), Bellcore established a standard for connecting one optical fiber component or system to another. That standard is officially named the “Synchronous Optical Network,” but is more commonly called “SONET.” The international version of the standard is officially named the “Synchronous Digital Hierarchy,” but it is more commonly called “SDH.”
Although differences exist between SONET and SDH, those differences are mostly in terminology. In most respects, the two standards are the same and, therefore, virtually all equipment that complies with either the SONET standard or the SDH standard also complies with the other. Therefore, for the purposes of this specification, the SONET standard and the SDH standard shall be considered interchangeable and the acronym/initialism “SONET/SDH” shall be defined as either the Synchronous Optical Network standard or the Synchronous Digital Hierarchy standard, or both.